NAME

pivot_root - change the root file system

SYNOPSIS

intpivot_root(constchar*new_root,constchar*put_old);

DESCRIPTION

pivot_root() moves the root file system of the calling process to the
directory put_old and makes new_root the new root file system of the
calling process.
The typical use of pivot_root() is during system startup, when the
system mounts a temporary root file system (e.g., an initrd), then
mounts the real root file system, and eventually turns the latter into
the current root of all relevant processes or threads.
pivot_root() may or may not change the current root and the current
working directory of any processes or threads which use the old root
directory. The caller of pivot_root() must ensure that processes with
root or current working directory at the old root operate correctly in
either case. An easy way to ensure this is to change their root and
current working directory to new_root before invoking pivot_root().
The paragraph above is intentionally vague because the implementation
of pivot_root() may change in the future. At the time of writing,
pivot_root() changes root and current working directory of each process
or thread to new_root if they point to the old root directory. This is
necessary in order to prevent kernel threads from keeping the old root
directory busy with their root and current working directory, even if
they never access the file system in any way. In the future, there may
be a mechanism for kernel threads to explicitly relinquish any access
to the file system, such that this fairly intrusive mechanism can be
removed from pivot_root().
Note that this also applies to the calling process: pivot_root() may or
may not affect its current working directory. It is therefore
recommended to call chdir("/") immediately after pivot_root().
The following restrictions apply to new_root and put_old:
- They must be directories.
- new_root and put_old must not be on the same file system as the
current root.
- put_old must be underneath new_root, that is, adding a non-zero
number of /.. to the string pointed to by put_old must yield the
same directory as new_root.
- No other file system may be mounted on put_old.
See also pivot_root(8) for additional usage examples.
If the current root is not a mount point (e.g., after chroot(2) or
pivot_root(), see also below), not the old root directory, but the
mount point of that file system is mounted on put_old.
new_root does not have to be a mount point. In this case, /proc/mounts
will show the mount point of the file system containing new_root as
root (/).

RETURNVALUE

On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.

ERRORS

pivot_root() may return (in errno) any of the errors returned by
stat(2). Additionally, it may return:
EBUSYnew_root or put_old are on the current root file system, or a
file system is already mounted on put_old.
EINVALput_old is not underneath new_root.
ENOTDIRnew_root or put_old is not a directory.
EPERM The calling process does not have the CAP_SYS_ADMIN capability.

VERSIONS

pivot_root() was introduced in Linux 2.3.41.

CONFORMINGTO

pivot_root() is Linux-specific and hence is not portable.

NOTES

Glibc does not provide a wrapper for this system call; call it using
syscall(2).

BUGS

pivot_root() should not have to change root and current working
directory of all other processes in the system.
Some of the more obscure uses of pivot_root() may quickly lead to
insanity.